What should be noted when using a pipette to dispense a solution?

Usage methods and precautions for pipettes. A pipette is a measuring instrument used solely to determine the volume of solution it dispenses. It is a long, thin glass tube with a bulged section in the middle. Its lower end is pointed, and a marking is engraved at the upper neck, serving as an indicator of the exact volume taken. Commonly used pipettes are available in sizes of 5, 10, 25, and 50 mL. A straight glass tube with scales is usually also called a pipette. Commonly used pipettes come in sizes of 1, 2, 5, and 10 mL, among others. The volumes transferred by pipettes and syringes can usually be accurate to 0.01 mL. 1. Before use: When using a pipette, first check its markings, accuracy class, the position of the scale markings, etc. Before using a pipette, it should be rinsed with chromic acid solution to remove oil residues from the inner wall of the tube. Then rinse the remaining wash solution with tap water, and clean it with distilled water. The inner wall of the pipette after washing should not have water droplets on it. Before transferring the solution, absorb the water inside and outside the tip of the pipette with filter paper first, then rinse the tube walls 2 to 3 times with the solution intended to be transferred, to ensure that the concentration of the transferred solution remains unchanged. 2. Liquid absorption: Use the thumb and middle finger of the right hand to hold the upper end of the pipette, and insert the lower end of the pipette into the solution from which liquid is to be drawn. It should not be inserted too shallowly or too deeply; generally, an insertion depth of 10–20 mm is appropriate. Inserting it too shallowly can result in vacuum formation, causing the solution to be drawn into the bulb and thus contaminating it, while inserting it too deeply can lead to excessive amount of solution adhering to the outside of the pipette. Hold the ear washer in the left hand, first expel the air from it, then attach its tip to the opening of the pipette. Gently release the compressed ear washer to allow the solution to be drawn into the tube – initially about 1/3 of the tube’s capacity. Use the index finger of the right hand to cover the tube opening, remove it, hold it horizontally, and rotate the tube so that the solution comes into contact with the area above the scale, thereby displacing any water on the inner wall. Then release the solution from the lower end of the tube and discard it. Repeat this process three times; after that, the solution can be drawn up to the mark on the tube. Immediately cover the tube opening again with the index finger of the right hand. 3. Adjust the liquid level: Lift the pipette upward away from the liquid surface, keeping the tip of the pipette against the inner wall of the container holding the solution; maintain an upright position for the pipette. Slightly relax the index finger (sometimes it is possible to rotate the pipette slightly) to allow the solution in the pipette to flow out slowly from the bottom opening, until the bottom of the meniscus is in contact with the marking. Immediately press the opening of the pipette tightly with the index finger. Remove the tip droplet against the wall, take it out of the pipette, and insert it into the container holding the receiving solution. 4. Pouring out the solution: If the container used to hold the solution is a conical flask, tilt the flask at 30°. Keep the pipette upright with its lower end pressed against the inner wall of the flask. Gently release your index finger to allow the solution to flow down slowly along the wall of the flask. After all the solution has been poured out, wait 15 seconds before removing the pipette, so that any solution remaining on its walls can flow out as well. If the pipette is not marked with the word “blow”, the solution remaining at the tip of the pipette cannot be blown out, as this residual solution is not included in the volume indicated by the pipette.

When using a pipette to dispense the solution, care should be taken not to shake it

Is there a problem with the third part you wrote? Does the outer wall of the pipette need to be wiped clean? ? Is it okay if the liquid level is not at the mark in the volumetric flask?

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